An optical spectrometer is a device that can analyze the. incoming light by frequency (wavelength) components and their intensities. In general, there are two types of spectrometers: dispersive and interferometric. A dispersive spectrometer has an optically dispersive component (e.g. prism or grating) to spatially disperse. the incoming light as a function of wavelength. The dispersed light is collected by multi-channel detectors. An interferometric spectrometer records the interferogram generated by the incoming light, and mathematically converts the interferogram to a spectrum. An example of an interferometric spectrometer is a Fourier-transform infrared spectrometer (FTIR) based on a Michelson interferometer.
Conventional optical spectrometers when used for imaging adopt a spot scanning process, wherein a single spot of light is illuminated on the surface of the surface of the sample for collecting multiwavelength optical data and/or image data, such as Raman or fluorescence data. The single spot of laser light on a surface of the sample of a conventional spot scanning spectrometer has a diameter of about 200 nanometers to 1 micron, when the spectrometer is used for imaging. When the sample is large, such as a biomolecular sample, spot scanning can only be performed at significantly limited speeds. This is because state-of-the-art spot scanning methods involve sample positioning after a spectrum is collected from each spot, which delays the scanning when many (e.g. >1000) spots need to be scanned for imaging application.
FIG. 1 shows the flow diagram of the conventional spot scanning system. A sample is positioned for measurement of an optical property at 102. A spectrum including a range of wavelengths or wavenumbers is collected at 104 typically by scanning through the range in more or less narrow increments. When the last position has not yet been reached at 106, then the process continues at 108 by collecting a spectrum for a next spot by repeating 104 for as many spots as may exist on the surface of an array.
FIG. 2 illustrates schematically a conventional single spot scanning system. An array 202 includes multiple pads 204. An array 202 may include more than 100 spots, particularly when a detailed image of the sample is desired. A sample beam 206 is illustrated with a dotted line in FIG. 2 transmitting a dichroic filter 208 and focusing lens 210 onto the spot 204. A Raman signal, fluorescence signal or other optical signal 212 emitted from the sample as a result of the illumination by the beam 206 is collected by first transmitting the lens 210 and reflecting from the dichroic filter 208. The signal 212 is then focused by a lens 214 to a spectrograph (e.g. Czerny-Terner spectrograph) and detector system 216. The sample is moved in two directions during the course of a complete scan as illustrated by the two bi-directional arrows 218. Alternatively, the laser spot on the sample can be moved instead of the sample by a scanning mirror (not shown) similar to the method used in confocal scanning.
Line scanning may be used, as illustrated at FIG. 3, in a faster process than the spot scanning technique of illustrated at FIGS. 1 and 2. A linear illumination 304 of the sample 202 is provided by a beam 306 that is passed through a cylindrical lens 307, filter 308 and lens 310. A linear emission 312 from the sample surface 302 is then collected by passing through focusing lens 310, reflecting from filter 308, transmitting lens 314 and being incident at linear imaging spectrograph (e.g. Czerny-Terner) and two-dimensional array detector 316. The sample is only moved in one direction, as illustrated by the single bidirectional arrow 318, in order to achieve an image.
A single channel Fourier-transform (FT) Raman spectrometry may be performed using a Thermo Nicolet™ system. This instrument includes a single channel detector and performs spot scanning in accordance with FIG. 1.